ICBM (intercontinental ballistic missile) address or missile address is hacker slang for one's longitude and latitude (preferably to seconds-of-arc accuracy) when placed in a signature or another publicly available file.
The form that used to register a site with the Usenet mapping project, before Internet connectivity became commonplace for Usenet sites, included a field for longitude and latitude, preferably to seconds-of-arc accuracy. This was actually used for generating geographically-correct maps of Usenet links on a plotter; however, it became traditional to refer to this as one's ICBM address or missile address, and some people include it in their sig block with that name (but a real missile address would include target elevation).
A typical tag might read "ICBM Address: 36.8274040 N, 108.0204086 W".
Today, using the ICBM method of coordinates is one method of geotagging webpages or other online material. Some projects parse ICBM address included in webpages via meta tags, which can then be used to map out sites added to its database.
This article is based in part on the Jargon File, which is in the public domain.